1. Field of the Invention
The invention relates generally to air thrust propeller driven boats. More particularly, the invention relates to a drive train for an air thrust propeller driven boat.
2. Description of the Related Art
Air thrust propeller driven boats, or "air boats" as they are commonly known, have found wide application and acceptance in areas where shallow water, reeds, everglades, partially frozen lakes and sub-surface debris present hazards to the operation of a submersed propeller. A significant advantage associated with air boats is their ability to hydroplane over the water's surface, and therefore to require no significant water depth for effective operation.
An air boat operates generally on the principle that once a certain speed is reached, the hull of the boat hydroplanes over the surface of the water, having relatively little impact on whatever lies beneath the surface of the water. Reaching the speed necessary for hydroplaning, or "getting up on the plane", as it is generally termed in the vernacular, requires a significant amount of power depending on the size and weight of the boat.
Typically, an air boat is powered by an aircraft engine mounted high above the water line of the boat hull to provide adequate clearance for the rotating propeller. Aircraft engines used in air boats are generally expensive, heavy, extremely powerful, noisy and generally have poor fuel economy. Further, the propeller is normally directly coupled to the engine crankshaft. There are several disadvantages to this configuration.
The propeller, when coupled directly to the crankshaft, rotates whenever the engine is running, generally making engine idling without forward motion difficult due to the continued air thrust being produced. Also, having a direct coupling between the propeller and the engine limits the boat's mobility in that only forward motion is possible. Further, because of the elevated mounting f the engine above the hull water line, the center of gravity of the air boat is high and rearward, making the boat unstable and unsafe under many operating conditions.
The typical propeller used in air boats is a two blade aircraft propeller not specifically designed for use in a marine application. The blade angle and curvature is not necessarily the most efficient, quietest or safest design for marine applications. Further, aircraft propellers are relatively long measured from blade tip to blade tip. When the propeller is rotating at maximum speed, the tip generates shock waves which produce a significant amount of noise making conversation between passengers virtually impossible, and fatiguing the operator.
Air boat propellers are usually protected by an open wire mesh cage shroud that serves to protect boaters and fowl from the dangers associated with a rotating propeller. The random momentum of incoming air on the upstream side of a propeller has a significant effect on the efficiency of the propeller. The typical propeller shroud does little to improve the efficiency of the propeller with respect to the incoming air.
The maintenance of an air boat having an aircraft engine presents considerable problems for the owner and/or operator. Aircraft engines are generally expensive to repair and aircraft mechanics are somewhat reluctant to work on a marine craft. Parts for aircraft engines are generally more expensive than comparable parts used in automotive and marine applications.
Steering of air boats has been accomplished by rudders of either the conventional submerged type or the aircraft type mounted in the slipstream of the propeller. The operation of submerged rudders is objectionable because any submerged movable part is subject to hazards such as sub-surface debris, rocks, etc. The use of a slipstream-mounted rudder is disclosed in U.S. Pat. No. 4,015,555. The use of such rudders can be disadvantageous since they are inefficient and are characterized by sluggish response because they are dependent upon the deflection of air.
Another method of steering an air thrust propeller driven boat consists of changing the direction of thrust by turning both engine and propeller together about a vertical axis, as disclosed in, for example, U.S. Pat. No. 4,005,673. This method is undesirable because of the instability associated with the elevated mounting of a heavy engine. Steering by turning both the propeller and engine as a unit has the additional disadvantage of offering no inherent self-centering characteristics. Once turned, this type of steering mechanism will continue to turn the boat until such time as the operator returns the mechanism to a centered position. The lack of a self-centering characteristic makes following a set course tiresome for the operator.
Yet another method of steering an air thrust propeller driven boat consists of an engine coupled via a combination of shafts and gears to a pivoting propeller support structure, as shown in U.S. Pat. No. 2,341,911. This design is disadvantageous due to its inherent complexity and because the vertical drive shaft coupling the engine and propeller may tend to induce rotation in the pivoting propeller support structure.
In general an air boat having an elevated engine mounted in the stern or rear of the hull can be difficult to control when there is a loss of power at high speed operation. At high speed, after engine shutdown or failure, the center of gravity of the boat, which is typically both high nd rearward, will continue in a forward direction with the boat. The hull of the boat, due to air resistance at high speeds and due to increasing water friction as the boat lowers into the water from a hydroplaning position, may tend to turn broadside, thus exposing one side of the boat to increased water resistance, thereby causing the boat to roll over and possibly injure passengers or the operator.